Semiconductor nanoparticle-based materials

ABSTRACT

In various embodiment, a primary particle includes a primary matrix material containing a population of semiconductor nanoparticles, with each primary particle further comprising an additive to enhance the physical, chemical and/or photo-stability of the semiconductor nanoparticles. A method of preparing such particles is described. Composite materials and light-emitting devices incorporating such primary particles are also described.

RELATED APPLICATIONS

This application claims the benefit of and priority to co-pendingapplication GB 0916699.2 filed Sep. 23, 2009 and U.S. Provisional PatentApplication Ser. No. 61/246,287 filed Sep. 28, 2009, the disclosures ofwhich are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

In various embodiments, the present invention relates to semiconductornanoparticle-based materials, particularly, but not exclusively, quantumdot-containing beads for use in the fabrication of quantum dot-basedlight-emitting devices.

There has been substantial interest in exploiting the properties ofcompound semiconductors consisting of particles with dimensions in theorder of 2-50 nm, often referred to as quantum dots (QDs) ornanocrystals. These materials are of commercial interest due to theirsize-tuneable electronic properties which can be exploited in manycommercial applications such as optical and electronic devices and otherapplications that range from biological labelling, photovoltaics,catalysis, biological imaging, LEDs, general space lighting andelectroluminescent displays amongst many new and emerging applications.

The most studied of semiconductor materials have been the chalcogenidesII-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSedue to its tuneability over the visible region of the spectrum.Reproducible methods for the large scale production of these materialshave been developed from “bottom up” techniques, whereby particles areprepared atom-by-atom, i.e., from molecules to clusters to particles,using “wet” chemical procedures.

Two fundamental factors, both related to the size of the individualsemiconductor nanoparticle, are responsible for their unique properties.The first is the large surface to volume ratio; as a particle becomessmaller, the ratio of the number of surface atoms to those in theinterior increases. This leads to the surface properties playing animportant role in the overall properties of the material. The secondfactor being, with many materials including semiconductor nanoparticles,that there is a change in the electronic properties of the material withsize; moreover, because of quantum confinement effects the band gapgradually becomes larger as the size of the particle decreases. Thiseffect is a consequence of the confinement of an ‘electron in a box’giving rise to discrete energy levels similar to those observed in atomsand molecules, rather than a continuous band as observed in thecorresponding bulk semiconductor material. Thus, for a semiconductornanoparticle, because of the physical parameters, the electron and hole,produced by the absorption of electromagnetic radiation (a photon, withenergy greater than the first excitonic transition), are closer togetherthan they would be in the corresponding macrocrystalline material,moreover the Coulombic interaction cannot be neglected. This leads to anarrow bandwidth emission, which is dependent upon the particle size andcomposition of the nanoparticle material. Thus, quantum dots have higherkinetic energy than the corresponding macrocrystalline material andconsequently the first excitonic transition (band gap) increases inenergy with decreasing particle diameter.

Core semiconductor nanoparticles, which consist of a singlesemiconductor material along with an outer organic passivating layer,tend to have relatively low quantum efficiencies due to electron-holerecombination occurring at defects and dangling bonds situated on thenanoparticle surface which can lead to non-radiative electron-holerecombinations. One method to eliminate defects and dangling bonds onthe inorganic surface of the quantum dot is to grow a second inorganicmaterial, having a wider band-gap and small lattice mismatch to that ofthe core material epitaxially on the surface of the core particle, toproduce a “core-shell” particle. Core-shell particles separate anycarriers confined in the core from surface states that would otherwiseact as non-radiative recombination centres. One example is a ZnS shellgrown on the surface of a CdSe core. Another approach is to prepare acore-multi shell structure where the electron-hole pair is completelyconfined to a single shell layer consisting of a few monolayers of aspecific material such as a quantum dot-quantum well structure. Here,the core is of a wide band gap material, followed by a thin shell ofnarrower band gap material, and capped with a further wide band gaplayer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on thesurface of the core nanocrystal to deposit just a few monolayers of HgSwhich is then over grown by a monolayer of CdS. The resulting structuresexhibit clear confinement of photo-excited carriers in the HgS layer. Toadd further stability to quantum dots and help to confine theelectron-hole pair one of the most common approaches is by epitaxiallygrowing a compositionally graded alloy layer on the core; this can helpto alleviate strain that could otherwise led to defects. Moreover for aCdSe core, in order to improve structural stability and quantum yield,rather growing a shell of ZnS directly on the core a graded alloy layerof Cd_(1-x)Zn_(x)Se_(1-y)S_(y) can be used. This has been found togreatly enhance the photoluminescence emission of the quantum dots.

Doping quantum dots with atomic impurities is an efficient way also ofmanipulating the emission and absorption properties of the nanoparticle.Procedures for doping of wide band gap materials, such as zinc selenideand zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), havebeen developed. Doping with different luminescence activators in asemiconducting nanocrystal can tune the photoluminescence andelectroluminescence at energies even lower than the band gap of the bulkmaterial whereas the quantum size effect can tune the excitation energywith the size of the quantum dots without having a significant change inthe energy of the activator-related emission.

The widespread exploitation of quantum dot nanoparticles has beenrestricted by their physical/chemical instability and incompatibilitywith many of the materials and/or processes required to exploit thequantum dots to their full potential, such as incorporation intosolvents, inks, polymers, glasses, metals, electronic materials,electronic devices, bio-molecules and cells. Consequently, a series ofquantum dot surface modification procedures has been employed to renderthe quantum dots more stable and compatible with the materials and/orprocessing requirements of a desired application.

A particularly attractive potential field of application for quantumdots is in the development of next generation light-emitting diodes(LEDs). LEDs are becoming increasingly important in modern-day life andit is envisaged that they have the potential to become one of the majorapplications for quantum dots, in for example, automobile lighting,traffic signals, general lighting, liquid crystal display (LCD)backlighting and display screens.

Currently, LED devices are made from inorganic solid-state compoundsemiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), andAlGaInN (green-blue); however, using a mixture of the availablesolid-state compound semiconductors, solid-state LEDs which emit whitelight cannot be produced. Moreover, it is difficult to produce “pure”colours by mixing solid-state LEDs of different frequencies. Therefore,currently the main method of colour mixing to produce a required colour,including white, is to use a combination of phosphorescent materialswhich are placed on top of the solid-state LED whereby the light fromthe LED (the “primary light”) is absorbed by the phosphorescent materialand then re-emitted at a different frequency (the “secondary light”),i.e., the phosphorescent materials down-convert the primary light to thesecondary light. Moreover, the use of white LEDs produced by phosphordown-conversion leads to lower cost and simpler device fabrication thana combination of solid-state red-green-blue LEDs.

Current phosphorescent materials used in down-converting applicationsabsorb UV or mainly blue light and convert it to longer wavelengths,with most phosphors currently using trivalent rare-earth doped oxides orhalophosphates. White emission can be obtained by blending phosphorswhich emit in the blue, green and red regions with that of a blue orUV-emitting solid-state device. i.e., a blue-light-emitting LED plus agreen phosphor such as, SrGa₂S₄:Eu₂ ⁺, and a red phosphor such as,SrSiEu₂ ⁺ or a UV-light-emitting LED plus a yellow phosphor such as,Sr₂P₂O₇:Eu₂ ⁺; Mu₂ ⁺, and a blue-green phosphor. White LEDs can also bemade by combining a blue LED with a yellow phosphor, however, colourcontrol and colour rendering is poor when using this methodology due tolack of tunability of the LEDs and the phosphor. Moreover, conventionalLED phosphor technology uses down-converting materials that have poorcolour rendering (i.e., colour rendering index (CRI)<75).

Rudimentary quantum dot-based light-emitting devices have been made byembedding colloidally produced quantum dots in an optically clear (orsufficiently transparent) LED encapsulation medium, typically a siliconeor an acrylate, which is then placed on top of a solid-state LED. Theuse of quantum dots potentially has some significant advantages over theuse of the more conventional phosphors, such as the ability to tune theemission wavelength, strong absorption properties and low scattering ifthe quantum dots are mono-dispersed.

For the commercial application of quantum dots in next-generationlight-emitting devices, the quantum dots may be incorporated into theLED encapsulating material while remaining as fully mono-dispersed aspossible and without significant loss of quantum efficiency. The methodsdeveloped to date are problematic, not least because of the nature ofcurrent LED encapsulants. Quantum dots can agglomerate when formulatedinto current LED encapsulants, thereby reducing the optical performanceof the quantum dots. Moreover, even after the quantum dots have beenincorporated into the LED encapsulant, oxygen can still migrate throughthe encapsulant to the surfaces of the quantum dots, which can lead tophoto-oxidation and, as a result, a drop in quantum yield (QY).

In view of the significant potential for the application of quantum dotsacross such a wide range of applications, including but not limited to,quantum dot-based light-emitting devices, work has already beenundertaken by various groups to try to develop methods to increase thestability of quantum dots so as to make them brighter, more long-livedand/or less sensitive to various types of processing conditions. Forexample, reasonably efficient quantum dot-based light-emitting devicescan be fabricated under laboratory conditions building on currentpublished methods; however, there remain significant challenges to thedevelopment of quantum dot-based materials and methods for fabricatingquantum dot-based devices, such as light-emitting devices, on aneconomically viable scale and which would provide sufficiently highlevels of performance to satisfy consumer demand.

SUMMARY

Various embodiments of the present invention obviate or mitigate one ormore of the above problems with semiconductor nanoparticle-basedmaterials and/or current methods for fabricating such materials.

A first aspect of embodiments of the present invention provides aprimary particle including a primary matrix material containing apopulation of semiconductor nanoparticles, wherein each primary particlefurther includes an additive to enhance the physical, chemical and/orphoto-stability of the semiconductor nanoparticles.

Embodiments of the current invention thus provide a means by which therobustness, and consequently, the performance of semiconductornanoparticles may be improved for use in a wide range of applications,particularly, but not exclusively the fabrication of semiconductornanoparticle-based light-emitting devices, preferably where the deviceincorporates an LED as a primary light source and the semiconductornanoparticles as a secondary light source. By providing each primaryparticle with one or more stability enhancing additives, thesemiconductor nanoparticles are less sensitive to their surroundingenvironment and subsequent processing steps.

In a preferred embodiment, a plurality of quantum dots are incorporatedinto one or more silica beads which also include a free-radialscavenger, such as benzophenone or a derivative thereof, which quantumdot-containing beads are then embedded or entrapped within a host LEDencapsulation material such as a silicone, an epoxy resin, a(meth)acrylate or a polymeric material. Such an arrangement is depictedschematically in FIG. 1, wherein an LED 1, which is arranged to emitblue primary light 2 upon the application of current, is submerged in acommercially available LED encapsulant 3 in which is embedded aplurality of quantum dot-containing silica beads 4, 5 also incorporatinga free-radial scavenger to enhance the stability of the beads; some ofthe beads 4 contain quantum dots that emit red secondary light 6 uponexcitation by the blue primary light from the LED 1, and the remainingbeads 4 contain quantum dots which emit green secondary light 7 uponexcitation by the blue primary light from the LED 1.

The term “bead” is used herein for convenience and is not intended toimpose any particular size or shape limitation to the material describedas a “bead”. Thus, for example, the beads may be spherical but otherconfigurations are possible. Where reference is made herein to“microbeads” this is intended to refer to “beads” as defined abovehaving a dimension on the micron scale.

The or each primary particle may be provided with a separate layer of asurface coating material. The term “coating” is used herein to refer toone or more layers of material provided on another material, which maypartially or fully cover the exterior surface or solvent accessiblesurface of that other material. The material of the “coating” maypenetrate at least partially into the internal structure of the materialto which it has been applied, provided the coating still affords a levelof protection or functions in some way as a barrier to the passage ofpotentially harmful species, e.g., oxygen, through the coated material.It will be appreciated from the wording used to define the variousaspects of the present invention herein that the coating applied to eachprimary particle results in the production of a plurality of separate,distinct coated particles rather than a plurality of particles containedor encapsulated within the same, unitary matrix-type material, such as aplurality of resin beads dispersed throughout an LED encapsulant.

The nanoparticle-containing primary particles or beads are preferablyprovided in the form of microbeads. By pre-loading small microbeads,which may range in size from 50 nm to 500 μm or more preferably 25 nm to0.1 mm or more preferably still 20 nm to 0.5 mm in diameter, withquantum dots, adding an additive, and then optionally providing asurface coating of, for example, a polymer or oxide material, theresulting beads are more stable towards their surrounding environmentand/or subsequent processing conditions, such as the incorporation ofthe quantum dot-containing beads into an LED encapsulation material on aUV or blue LED. As a result, not only does handling of the quantum dotsbecome easier, but their optical performance may be improved and it maybecome simpler to tune the colour of the light they emit, for examplewhen used in an LED-based device. Moreover, this approach is simplerthan attempting to incorporate quantum dots directly into an LEDencapsulate (for example, a silicone, an epoxy, a (meth)acrylate, apolymeric material or the like) in terms of ease of colour rendering,processing, and reproducibility and offers greater quantum dot stabilityto photo-oxidation.

The quantum dot-containing beads may be made to any desirable size, suchas the same size as currently employed YAG phosphor materials whichrange from 10 to 100 μm and may thus be supplied to existing LEDmanufacturers in a similar form to that of the current commercially usedphosphor materials. Moreover, the quantum dot-containing beadsincorporating the additive(s) is (are) in a form that is compatible withthe existing LED fabrication infrastructure.

With the advantage of very little or no loss of quantum dot quantumyield (QY) in processing; this new approach of optionally coated quantumdot-containing beads incorporating stability-enhancing additives leadsto less loss of quantum efficiency than when formulating the quantumdots directly into a LED encapsulation medium or when using quantum dotbeads not incorporating such additives or a protective surface coating.Because there is very little or no loss of quantum yield it is easier tocolour render and less binning may be needed. It has been shown thatwhen formulating quantum dots directly into an encapsulation mediumusing prior art methods, colour control is very difficult due to quantumdot re-absorption or loss of quantum yield and shifting of the PL maxposition. Moreover batch to batch, i.e., device-to-device,reproducibility is very difficult or impossible to achieve. Bypre-loading the quantum dots into one or more beads also incorporatingthe stability-enhancing additive(s), and then optionally coating thebeads, the colour of the light emitted by the device is of higherquality, easier to control and is more reproducible.

By incorporating known amounts of quantum dots into beads alsoincorporating stability-enhancing additives, and optionally providingthe beads with a protective surface coating, migration of deleteriousspecies, such as moisture, oxygen and/or free radicals, is eliminated orat least reduced, thereby eliminating or at least minimising thesecommon hurdles to the industrial production of quantum dot basedmaterials and devices.

A second aspect of some embodiments of the present invention provides amethod for preparing a primary particle including a primary matrixmaterial, a population of semiconductor nanoparticles and an additive toenhance the physical, chemical and/or photo-stability of thesemiconductor nanoparticles, the method including combining thesemiconductor nanoparticles, primary matrix material and additive underconditions suitable to produce the primary particle.

An additive may be combined with “naked” semiconductor nanoparticles andprecursors to the primary matrix material during initial production ofthe primary particles. Alternatively, or additionally, an additive maybe added after the semiconductor nanoparticles have been entrappedwithin the primary matrix material.

The quantum-dot containing primary particles incorporating an additivemay be dispersed in a secondary matrix material, which may be the sameor different to the primary matrix material.

A further aspect of embodiments of the present invention provides acomposite material incorporating a plurality of primary particlesaccording to the first aspect of the present invention dispersed withina secondary matrix material.

A still further aspect provides a light-emitting device including aprimary light source in optical communication with a formulationincluding a composite material according to the above further aspectembedded in a host light-emitting diode encapsulation medium.

The secondary matrix material may be selected from the group of primarymatrix materials set out above. By way of example, the secondary matrixmaterial may include a material selected from the group consisting of apolymer, a resin, a monolith, a glass, a sol gel, an epoxy, a siliconeand a (meth)acrylate.

Additionally, the secondary matrix material may be formed into one ormore secondary particles containing one or more primary particles. Thesecondary particles may be provided with a further additive in a similarmanner to that described herein in respect of additives added to theprimary particles. Accordingly, the secondary matrix material may be inthe form of one or more secondary particles and the or each secondaryparticle may be provided with a further stability-enhancing additive,which may be the same or different to the one or more additives presentin the primary particles.

Alternatively, the quantum dots may first be captured within one or moretypes of matrix material, such as one or more types of polymeric bead,and then each of those beads, or beads within beads, may be containedwithin a primary matrix material to form the primary particles of thefirst and second aspects of the present invention, which incorporate astability-enhancing additive. Thus, the semiconductor nanoparticlescontained within the primary matrix material may be “naked”nanoparticles, or may already be contained within one or more layers ofmatrix material before being captured within the primary matrix materialand coated.

FIGS. 7 to 10 depict processes according to four preferred embodimentsof the present invention in which additives are added at differentstages during the formation of quantum dot-containing beads, or quantumdot-containing beads within one or more types of larger bead.

FIG. 7 illustrates a process according to a first embodiment of thepresent invention wherein an additive is combined with a population of“naked” quantum dots during formation of a primary particle containingthe quantum dots and, consequently, the additive. FIG. 8 illustrates aprocess according to a second embodiment wherein “naked” quantum dotsare first encapsulated within a bead formed of a first type of polymer(polymer 1) and then an additive is combined with the quantumdot-containing bead during formation of a primary particle made from asecond type of polymer (polymer 2) containing the quantum dot-containingbead and, consequently, the additive. FIG. 9 depicts a process accordingto a third embodiment wherein quantum dots are first encapsulated withina population of beads formed of a first type of polymer (polymer 1) andthen an additive is combined with the quantum dot-containing beadsduring formation of a primary particle made from a second type ofpolymer (polymer 2) containing the quantum dot-containing beads and,consequently, the additive. FIG. 10 illustrates a process according to afourth embodiment wherein quantum dots are first encapsulated within apopulation of beads formed of a first type of polymer (polymer 1), whichare then encapsulated within a bead formed of a second type of polymer(polymer 2) to form a “bead-in-bead” composite material, and then anadditive is combined with the quantum dot-containing bead-in-beadcomposite material during formation of a primary particle made from athird type of polymer (polymer 3) containing the quantum dot-containing“bead-in-bead” composite material and, consequently, the additive. Itwill be appreciated that any of the above embodiments may be combinedsuch that additives could be added at more than one stage during primaryparticle formation, resulting in primary particles containingbead-in-bead composites with the same or different additives in two ormore layers or shells of the primary particles.

A further aspect of embodiments of the present invention provides alight-emitting device including a primary light source in opticalcommunication with a formulation including a plurality of primaryparticles according to the first aspect of the present inventionembedded in a host light-emitting diode encapsulation medium.

BRIEF DESCRIPTION OF FIGURES

Embodiments of the present invention are illustrated with reference tothe following non-limiting examples and figures in which:

FIG. 1 schematically depicts a quantum dot-based light-emitting deviceaccording to an aspect of embodiments of the present invention;

FIG. 2 is a 2° CIE 1931 chromaticity diagram;

FIG. 3 is a 2° CIE 1931 colour matching diagram matching functions x, y,z;

FIG. 4 is a schematic representation of an additive-containingQD-bead-based light-emitting device employing multi-coloured, multiplequantum dot types in each bead such that each bead emits white secondarylight;

FIG. 5 is a schematic representation of an additive-containingQD-bead-based light-emitting device employing multi-coloured, multiplequantum dot types in different beads such that each bead contains asingle quantum dot type emitting a single colour, a mixture of the beadscombining to produce white secondary light;

FIG. 6 is a schematic representation of an additive-containingQD-bead-based light-emitting device employing a singly coloured, singlequantum dot type in all beads such that a mixture of the beads emits asingle colour of secondary light (in this case, red light);

FIG. 7 is a schematic representation of a process according to a firstembodiment of the present invention wherein an additive is combined witha population of quantum dots during formation of a primary particlecontaining the quantum dots and, consequently, the additive;

FIG. 8 is a schematic representation of a process according to a secondembodiment of the present invention wherein quantum dots are firstencapsulated within a bead formed of a first type of polymer (polymer 1)and then an additive is combined with the quantum dot-containing beadduring formation of a primary particle made from a second type ofpolymer (polymer 2) containing the quantum dot-containing bead and,consequently, the additive;

FIG. 9 is a schematic representation of a process according to a thirdembodiment of the present invention wherein quantum dots are firstencapsulated within a population of beads formed of a first type ofpolymer (polymer 1) and then an additive is combined with the quantumdot-containing beads during formation of a primary particle made from asecond type of polymer (polymer 2) containing the quantum dot-containingbeads and, consequently, the additive;

FIG. 10 is a schematic representation of a process according to a fourthembodiment of the present invention wherein quantum dots are firstencapsulated within a population of beads formed of a first type ofpolymer (polymer 1), which are then encapsulated within a bead formed ofa second type of polymer (polymer 2), and then an additive is combinedwith the quantum dot-containing beads during formation of a primaryparticle made from a third type of polymer (polymer 3) containing thequantum dot-containing beads and, consequently, the additive;

FIG. 11 is a schematic representation of a population of quantum dotsentrapped within a primary particle in the form of a polymer beadaccording to a preferred embodiment of the present invention in whichthe primary particle is provided with a surface coating of an inorganicmaterial, and the primary particles are dispersed within a secondarymatrix material in the form of an LED encapsulant disposed on an LED toprovide a light-emitting device according to a preferred embodiment ofthe present invention;

FIG. 12 is a schematic representation of a population of quantum dotsentrapped within a primary particle in the form of a polymer bead madefrom a first type of polymer (polymer 1) which is encapsulated within asecond type of polymer material (polymer 2) which is provided with asurface coating of an inorganic material according to a preferredembodiment of the present invention, and the encapsulated primaryparticles are dispersed within a secondary matrix material in the formof an LED encapsulant disposed on an LED to provide a light-emittingdevice according to a preferred embodiment of the present invention;

FIG. 13 is a schematic representation of a population of quantum dotsentrapped within a population of primary particles in the form ofpolymer beads (bead 1) according to a preferred embodiment of thepresent invention in which each of the primary particles is providedwith a surface coating of an inorganic material, before the coatedprimary particles are dispersed within a second type of bead (bead 2) toproduce a “bead-in-bead” composite material, and then the bead-in-beadcomposite material is dispersed within a secondary matrix material inthe form of an LED encapsulant disposed on an LED to provide alight-emitting device according to a preferred embodiment of the presentinvention; and

FIG. 14 is a schematic representation of a population of quantum dotsentrapped within a population of primary particles in the form ofpolymer beads according to a preferred embodiment of the presentinvention, the population of primary particles being dispersed within asecond type of bead to produce a “bead-in-bead” composite material whichis then provided with an inorganic surface coating layer, and then thebead-in-bead composite material is dispersed within a secondary matrixmaterial in the form of an LED encapsulant disposed on an LED to providea light-emitting device according to a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION

Primary Matrix Material

The primary matrix material is preferably an optically transparentmedium, i.e., a medium through which light may pass, and which may be,but does not have to be substantially optically clear. The primarymatrix material, preferably in the form of a bead or microbead, may be aresin, polymer, monolith, glass, sol gel, epoxy, silicone,(meth)acrylate or the like, or may include silica.

Examples of preferred primary matrix materials include acrylate polymers(e.g., polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycoldimethacrylate, polyvinylacetate etc), epoxides (e.g., EPOTEK 301 A+BThermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), silica, silica-acrylate hybrids,polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polydivinylbenzene, polyethylene, polypropylene, polyethylene terephthalate (PET),polyisobutylene (butyl rubber), polyisoprene, and cellulose derivatives(methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Stability-Enhancing Additives

The additives which may be added singly or in any desirable combinationto the primary particles containing the semiconductor nanoparticles maybe grouped according to their intended function as follows:

-   -   a. Mechanical sealing: Fumed silica (e.g., Cab-O-Sil™), ZnO,        TiO₂, ZrO, Mg stearate, Zn Stearate, all used as a filler to        provide mechanical sealing and/or reduce porosity;    -   b. Capping agents: Tetradecyl phosphonic acid (TDPA), oleic        acid, stearic acid, polyunsaturated fatty acids, sorbic acid. Zn        methacrylate, Mg stearate, Zn Stearate, isopropyl myristate.        Some of these have multiple functionality and may act as capping        agents, free radical scavengers and/or reducing agents;    -   c. Reducing agents: Ascorbic acid palmitate, alpha tocopherol        (vitamin E), octane thiol, butylated hydroxyanisole (BHA),        butylated hydroxytoluene (BHT), gallate esters (propyl, lauryl,        octyl and the like), and a metabisulfite (e.g., the sodium or        potassium salt);    -   d. Free radical scavengers: benzophenones; and    -   e. Hydride reactive agents: 1,4-butandiol, 2-hydroxyethyl        methacrylate, allyl methacrylate, 1,6 heptadiene-4-ol, 1,7        octadiene, and 1,4 butadiene.

The selection of the additive or additives for a particular applicationdepends upon the nature of the semiconductor nanoparticle material(e.g., how sensitive the nanoparticle material is to physical, chemicaland/or photo-induced degradation), the nature of the primary matrixmaterial (e.g., how porous it is to potentially deleterious species,such as free-radicals, oxygen, moisture etc), the intended function ofthe final material or device which will contain the primary particles(e.g., the operating conditions of the material or device), and theprocess conditions for fabricating the final material or device. Thus,with prior knowledge of the above risk-factors, one or more appropriateadditives may be selected from the above five lists to suit anydesirable semiconductor nanoparticle application.

Primary Particle Surface Coating Materials

One of the intended functions of the coating which may be provided onthe primary particles is to provide each primary particle with aprotective barrier to prevent the passage or diffusion of potentiallydeleterious species, e.g., oxygen, moisture or free radicals, from theexternal environment through the primary matrix material to thesemiconductor nanoparticles. As a result, the semiconductornanoparticles are less sensitive to their surrounding environment andthe various processing conditions typically required to utilise thenanoparticles in applications such as the fabrication of LED-basedlight-emitting devices.

The coating is preferably a barrier to the passage of oxygen or any typeof oxidising agent through the primary matrix material. The coating maybe a barrier to the passage of free radical species through the primarymatrix material, and/or is preferably a moisture barrier so thatmoisture in the environment surrounding the primary particles cannotcontact the semiconductor nanoparticles within the primary particles.

The coating may provide a layer of coating material on a surface of theprimary particle of any desirable thickness provided it affords thepreferred level of protection. The surface layer coating may be around 1to 10 nm thick, up to around 400 to 500 nm thick, or more. Preferredlayer thicknesses are in the range 1 nm to 200 nm, more preferablyaround 5 to 100 nm.

In a first preferred embodiment, the coating includes an inorganicmaterial, such as a dielectric (insulator), a metal oxide, a metalnitride or a silica-based material (e.g., a glass).

The metal oxide may be a single metal oxide (i.e., oxide ions combinedwith a single type of metal ion, e.g., Al₂O₃), or may be a mixed metaloxide (i.e., oxide ions combined with two or more types of metal ion,e.g., SrTiO₃). The metal ion(s) of the (mixed) metal oxide may beselected from any suitable group of the periodic table, such as group 2,13, 14 or 15, or may be a transition metal, d-block metal, or lanthanidemetal.

Preferred metal oxides are selected from the group consisting of Al₂O₃,B₂O₃, CO₂O₃, Cr₂O₃, CuO, Fe₂O₃, Ga₂O₃, HfO₂, In₂O₃, MgO, Nb₂O₅, NiO,SiO₂, SnO₂, Ta₂O₅, TiO₂, ZrO₂, Sc₂O₃, Y₂O₃, GeO₂, La₂O₃, CeO₂, PrO_(x)(x=appropriate integer), Nd₂O₃, Sm₂O₃, EuO_(y) (y=appropriate integer),Gd₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, SrTiO₃, BaTiO₃, PbTiO₃,PbZrO₃, Bi_(m)Ti_(n)O (m=appropriate integer; n=appropriate integer),Bi_(a)Si_(b)O (a=appropriate integer; b=appropriate integer), SrTa₂O₆,SrBi₂Ta₂O₉, YScO₃, LaAIO₃, NdAIO₃, GdScO₃, LaScO₃, LaLuO₃, Er₃Ga₅O₁₃.

Preferred metal nitrides may be selected from the group consisting ofBN, AlN, GaN, InN, Zr₃N₄, Cu₂N, Hf₃N₄, SiN_(c) (c=appropriate integer),TiN, Ta₃N₅, Ti—Si—N, Ti—Al—N, TaN, NbN, MoN, WN_(d) (d=appropriateinteger), and WN_(e)C_(f)(e=appropriate integer; f=appropriate integer).

The inorganic coating may include silica in any appropriate crystallineform.

The coating may incorporate an inorganic material in combination with anorganic or polymeric material. By way of example, in a preferredembodiment, the coating is an inorganic/polymer hybrid, such as asilica-acrylate hybrid material.

In a second preferred embodiment, the coating includes a polymericmaterial, which may be a saturated or unsaturated hydrocarbon polymer,or may incorporate one or more heteroatoms (e.g., O, S, N, halo) orheteroatom-containing functional groups (e.g., carbonyl, cyano, ether,epoxide, amide and the like).

Examples of preferred polymeric coating materials include acrylatepolymers (e.g., polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycoldimethacrylate, polyvinylacetate etc), epoxides (e.g., EPOTEK 301 A+BThermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), polyetheretherketone (PEEK),polyvinylidene fluoride (PVDF), polydivinyl benzene, polyethylene,polypropylene, polyethylene terephthalate (PET), polyisobutylene (butylrubber), polyisoprene, and cellulose derivatives (methyl cellulose,ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

By incorporating quantum dots into primary particle materials of thekind described above and coating the particles, he otherwise reactivequantum dots may be protected from the potentially damaging surroundingchemical environment. Moreover, by placing a number of quantum dots intoa single bead, for example in the size range from 20 nm to 500 μm indiameter, and providing the bead with a suitable protective coating of,for example, a polymeric or inorganic material, the resulting coatedQD-bead is more stable than either free “naked” quantum dots, oruncoated QD-beads to the types of chemical, mechanical, thermal and/orphoto-processing steps which are required to incorporate quantum dots inmost commercial applications, such as when employing quantum dots asdown-converters in a QD-solid-state-LED light-emitting device.

Each primary particle may contain any desirable number and/or type ofsemiconductor nanoparticles. Thus, the primary matrix material of theprimary particle may contain a single type of semiconductornanoparticle, e.g., InP, InP/ZnS or CdSe, of a specific size range, suchthat the plurality of QD-containing beads emits monochromatic light of apre-defined wavelength, i.e., colour. The colour of the emitted lightmay be adjusted by varying the type of semiconductor nanoparticlematerial used, e.g., changing the size of the nanoparticle, thenanoparticle core semiconductor material and/or adding one or more outershells of different semiconductor materials.

Moreover, colour control may also be achieved by incorporating differenttypes of semiconductor nanoparticles, for examples nanoparticles ofdifferent size and/or chemical composition within the primary matrixmaterial of each particle.

Furthermore, the colour and colour intensity may be controlled byselecting an appropriate number of semiconductor nanoparticles withineach particle. Preferably each primary particle contains at least around1000 semiconductor nanoparticles of one or more different types, morepreferably at least around 10,000, more preferably at least around50,000, and most preferably at least around 100,000 semiconductornanoparticles of one or more different types.

Where the primary particles are provided in the preferred form of beadsor microbeads, some or all of the beads preferably contain one or moresemiconductor nanoparticles capable of secondary light emission uponexcitation by primary light emitted by a primary light source (e.g., anLED).

The primary particles may be dispersed within an encapsulating medium,such as an LED encapsulant, to provide a robust QD-containingformulation which may then safely be used in subsequent processingsteps, for example, to deposit a desired amount of such a formulation onto an LED chip to provide an QD/LED-based light-emitting device. Anydesirable number of beads may be dispersed or embedded within theencapsulating medium; for example, the formulation may contain 1 to10,000 beads, more preferably 1 to 5000 beads, and most preferably 5 to1000 beads.

It should also be appreciated that the encapsulating medium may haveembedded therein one or more type of semiconductornanoparticle-containing primary particles. That is, two or moredifferent types of primary particles (one or more containing thenanoparticles) may be embedded within the same encapsulating medium. Inthis way, where the population of nanoparticles contains more than onedifferent type of nanoparticle, the nature of the primary particle maybe selected for optimum compatibility with both the different types ofnanoparticles and the particular medium used.

Advantages of quantum dot-containing beads incorporatingstability-enhancing additives, optionally also incorporating a surfacecoating, over free quantum dots or uncoated quantum dot-containing beadsmay include greater stability to air and moisture, greater stability tophoto-oxidation and/or greater stability to mechanical processing.Moreover, by pre-loading small microbeads, which may range in size froma few 50 nm to 500 μm, with quantum dots, adding a stability-enhancingadditive and optionally coating the individual microbeads prior toincorporating a plurality of such quantum dot-containing beads into anLED encapsulation material on a UV or blue LED, it is a relativelysimple process to change, in a controllable and reproducible manner, thecolour of the light emitted by the resulting LED-based light-emittingdevice.

Semiconductor Nanoparticles

Any desirable type of semiconductor nanoparticle may be employed inembodiments of the present invention. In a preferred embodiment, thenanoparticle contains ions, which may be selected from any desirablegroup of the periodic table, such as but not limited to group 11, 12,13, 14, 15 or 16 of the periodic table. The nanoparticles mayincorporate transition metal ions or d-block metal ions. It is preferredthat the nanoparticles contain first and second ions with the first ionpreferably selected from group 11, 12, 13 or 14 and the second ionpreferably selected from group 14, 15 or 16 of the periodic table. Thenanoparticles may contain one or more semiconductor materials selectedfrom the group consisting of CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe,InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe,Si, Ge, MgS, MgSe, MgTe and combinations thereof. Moreover, thenanoparticles may be binary, tertiary or quaternary core, core-shell orcore-multi shell, doped or graded nanoparticles.

Any appropriate method may be employed to produce the semiconductornanoparticles employed in the various aspects of the present invention.That being said, it is preferred that the semiconductor nanoparticles beproduced by converting a nanoparticle precursor composition to thematerial of the nanoparticles in the presence of a molecular clustercompound under conditions permitting seeding and growth of thenanoparticles on the cluster compound. The method may employ themethodology set out in the applicant's co-pending European patentapplication (publication No. EP1743054A) and U.S. Publication No.2007-0202333; both of these publications are incorporated herein byreference in their entireties.

Conveniently, the nanoparticles may incorporate first and second ionsand the nanoparticle precursor composition may include first and secondnanoparticle precursor species containing the first and second ionsrespectively which are combined, preferably in the presence of amolecular cluster compound, as exemplified below in Synthetic Methods1.1 and 1.2.

The first and second precursor species may be separate species in theprecursor composition or may form part of a single molecular speciescontaining both the first and second ions.

In the preferred embodiments employing a molecular cluster compound, itis preferred that the molecular clusters contain third and fourth ions.At least one of the third and fourth ions is preferably different fromthe first and second ions contained in the first and second nanoparticleprecursor species respectively. The third and fourth ions may beselected from any desirable group of the periodic table, such as but notlimited to group 11, 12, 13, 14, 15 or 16 of the periodic table. Thethird and/or fourth ion may be a transition metal ion or a d-block metalion. Preferably the third ion is selected from group 11, 12, 13 or 14and the fourth ion is selected from group 14, 15 or 16 of the periodictable.

By way of example, the molecular cluster compound may incorporate thirdand fourth ions from groups 12 and 16 of the periodic table respectivelyand the first and second ions derived from the first and secondnanoparticle precursor species may be taken from groups 13 and 15 of theperiodic table respectively as in Synthetic Method 1.2. The methodologydescribed in the applicant's International PCT patent application(application No. PCT/GB2008/002560) and U.S. Pat. No. 7,588,828 may beemployed; both of these references are incorporated herein by referencein their entireties.

It will be appreciated that during the reaction of the first and secondnanoparticle precursor species, the first nanoparticle precursor speciesmay be added in one or more portions and the second nanoparticleprecursor species may be added in one or more portions. The firstnanoparticle precursor species is preferably added in two or moreportions. In this case, it is preferred that the temperature of areaction mixture containing the first and second nanoparticle precursorspecies is increased between the addition of each portion of the firstprecursor species. Additionally or alternatively, the secondnanoparticle precursor species may be added in two or more portions,whereupon the temperature of a reaction mixture containing the first andsecond nanoparticle precursor species may be increased between theaddition of each portion of the second precursor species. Themethodology described in the applicant's co-pending European patentapplication (application No. 06808360.9) and US. Publication No.2007-0104865 may be used; both of these references are incorporatedherein by reference in their entireties.

The coordination about the final inorganic surface atoms in any core,core-shell or core-multi shell, doped or graded nanoparticle istypically incomplete, with highly reactive non-fully coordinated atomsacting as “dangling bonds” on the surface of the particle, which maylead to particle agglomeration. This problem is typically overcome bypassivating (capping) the “bare” surface atoms with protecting organicgroups.

In many cases, the capping agent is the solvent in which thenanoparticles have been prepared, and consists of a Lewis base compound,or a Lewis base compound diluted in an inert solvent such as ahydrocarbon. There is a lone pair of electrons on the Lewis base cappingagent that are capable of a donor type coordination to the surface ofthe nanoparticle; ligands of this kind include, but are not limited to,mono- or multi-dentate ligands such as phosphines (trioctylphosphine,triphenylphosphine, t-butylphosphine etc.), phosphine oxides(trioctylphosphine oxide, triphenylphosphine oxide etc.), alkylphosphonic acids, alkyl-amines (hexadecylamine, octylamine etc.),aryl-amines, pyridines, long chain fatty acids and thiophenes.

In addition to the outermost layer of organic material or sheathmaterial (capping agent) helping to inhibit nanoparticle-nanoparticleaggregation, this layer may also protect the nanoparticles from theirsurrounding electronic and chemical environments, and provide a means ofchemical linkage to other inorganic, biological or organic material,whereby the functional group is pointing away from the nanoparticlesurface and is available to bond/react/interact with other availablemolecules. Examples include, but are not limited to, amines, alcohols,carboxylic acids, esters, acid chloride, anhydrides, ethers, alkylhalides, amides, alkenes, alkanes, alkynes, allenes, amino acids,azides, groups etc. The outermost layer (capping agent) of a quantum dotmay also consist of a coordinated ligand that processes a functionalgroup that is polymerisable and may be used to form a polymer layeraround the nanoparticle. The outermost layer may also consist of organicunits that are directly bonded to the outermost inorganic layer such asvia a disulphide bond between the inorganic surface (e.g., ZnS) and athiol capping molecule. These may also possess additional functionalgroup(s), not bonded to the surface of the particle, which may be usedto form a polymer around the particle, or for furtherreaction/interaction/chemical linkage.

An example of a material to which nanoparticle surface binding ligandsmay be linked is the primary matrix material from which the primaryparticles are formed. There are a number of approaches to incorporatesemiconductor nanoparticles, such as quantum dots, into the types ofprimary matrix materials described hereinbefore by pre-coating thenanoparticles with ligands that are compatible in some way with thematrix material of the primary particles. By way of example, in thepreferred embodiment where the semiconductor nanoparticles are to beincorporated into polymeric beads, the nanoparticles may be produced soas to possess surface ligands which are polymerizable, hydrophobic,hydrophilic, positively or negatively charged, or functionalised with areactive group capable of associating with the polymer of the polymericbeads by chemical reaction, covalent linkage or non-covalent interaction(e.g., interchelation).

The inventors have determined that one may take quantum dots preparedusing any desirable method, incorporate these quantum dots into silicaor polymer beads also including at least one type of stability-enhancingadditive, and then optionally coat the beads with a protective barrierlayer of a material such as a polyacrylate or dielectric metal oxidelike aluminium oxide, to provide significantly more robust, easilyprocessible quantum dot-containing materials. Quantum dot-containingbeads of this kind may be employed in a wide range of applications,particularly, but not exclusively, the fabrication of LED-basedlight-emitting devices wherein the QD-beads are embedded within a hostLED encapsulant and then deposited onto a solid-state LED chip to form aquantum dot-based light-emitting device.

Incorporating Quantum Dots into Beads

Considering the initial step of incorporating quantum dots into beads, afirst option is to incorporate the quantum dots directly into the beads.A second option is to immobilise the quantum dots in beads throughphysical entrapment. These methods may be used to make a population ofbeads that contain just a single type of quantum dot (e.g., one colour)by incorporating a single type of quantum dot into the beads.Alternatively, beads that contain 2 or more types of quantum dots (e.g.,two or more colours) may be constructed by incorporating a mixture oftwo or more types of quantum dot (e.g., material and/or size) into thebeads. Such mixed beads may then be combined in any suitable ratio toemit any desirable colour of secondary light following excitation by theprimary light emitted by the primary light source (e.g., LED). This isexemplified in FIGS. 4 to 6 below which schematically show QD-beadlight-emitting devices including respectively: a) multi-coloured,multiple quantum dot types in each bead such that each bead emits whitesecondary light; b) multi-coloured, multiple quantum dot types indifferent beads such that each bead contains a single quantum dot typeemitting a single colour, a mixture of the beads combining to producewhite secondary light; and c) singly coloured, single quantum dot typein all beads such that a mixture of the beads emits a single colour ofsecondary light, e.g., red.

The or each stability-enhancing additive may be added to the quantumdot-containing beads during initial bead formation and/or after thebeads have been formed independently of which of the two options set outabove are employed to incorporate the quantum dots within the beads.

Incorporating Quantum Dots Beads During Bead Formation

With regard to the first option of incorporating the quantum dotsdirectly into the primary particles (i.e., the beads) during beadformation, by way of example, suitable core, core/shell orcore/multishell nanoparticles (e.g., InP/ZnS core/shell quantum dots)may be contacted by one or more bead precursors (e.g., an acrylatemonomer, a silicate material, or a combination of both) and thensubjected to suitable conditions (e.g., introduction of a polymerisationinitiator) to form the bead material. One or more stability-enhancingadditive may be included in the reaction mixture in which thenanoparticles are contacted by the bead precursors. Moreover, at thisstage, a surface coating may be applied to the beads.

By way of further example, hexadecylamine-capped CdSe-basedsemiconductor nanoparticles may be treated with at least one, morepreferably two or more polymerisable ligands (optionally one ligand inexcess) resulting in the displacement of at least some of thehexadecylamine capping layer with the polymerisable ligand(s). Thedisplacement of the capping layer with the polymerisable ligand(s) maybe accomplished by selecting a polymerisable ligand or ligands withstructures similar to that of trioctylphosphine oxide (TOPO), which is aligand with a known and very high affinity for CdSe-based nanoparticles.It will be appreciated that this basic methodology may be applied toother nanoparticle/ligand pairs to achieve a similar effect. That is,for any particular type of nanoparticle (material and/or size), one ormore appropriate polymerisable surface binding ligands may be selectedby choosing polymerisable ligands including a structural motif which isanalogous in some way (e.g., has a similar physical and/or chemicalstructure) to the structure of a known surface binding ligand. Once thenanoparticles have been surface-modified in this way, they may then beadded to a monomer component of a number of microscale polymerisationreactions to form a variety of quantum dot-containing resins and beads.

Examples of polymerisation methods that may be used to construct quantumdot-containing beads include, but are not limited to, suspension,dispersion, emulsion, living, anionic, cationic, RAFT, ATRP, bulk, ringclosing metathesis and ring opening metathesis. Initiation of thepolymerisation reaction may be caused by any appropriate means thatcauses the monomers to react with one another, such as free radicals,light, ultrasound, cations, anions, heat.

A preferred method is suspension polymerisation involving thermal curingof one or more polymerisable monomers from which the primary matrixmaterial is to be formed. The polymerisable monomers may, for example,include methyl (meth)acrylate, ethylene glycol dimethacrylate and/orvinyl acetate.

Quantum dot-containing beads may be generated simply by adding quantumdots to the mixture of reagents used to construct the beads. In someinstances quantum dots (nascent quantum dots) may be used as isolatedfrom the reaction employed to synthesise them and are thus generallycoated with an inert outer organic ligand layer. In an alternativeprocedure a ligand exchange process may be carried out prior to the beadforming reaction. Here one or more chemically reactive ligands (forexample this might be a ligand for the quantum dots which also containsa polymerisable moiety) is added in excess to a solution of nascentquantum dots coated in an inert outer organic layer. After anappropriate incubation time the quantum dots are isolated, for exampleby precipitation and subsequent centrifugation, washed and thenincorporated into the mixture of reagents used in the bead formingreaction/process.

Both quantum dot incorporation strategies result in statistically randomincorporation of the quantum dots into the beads and thus thepolymerisation reaction result in beads containing statistically similaramounts of the quantum dots and, optionally, the one or more additives.Bead size may be controlled by the choice of polymerisation reactionused to construct the beads, and additionally, once a polymerisationmethod has been selected, bead size may also be controlled by selectingappropriate reaction conditions, e.g., in a suspension polymerisationreaction by stirring the reaction mixture more quickly to generatesmaller beads. Moreover the shape of the beads may be readily controlledby choice of procedure in conjunction with whether or not the reactionis carried out in a mould. The composition of the beads may be alteredby changing the composition of the monomer mixture from which the beadsare constructed. Similarly the beads may also be cross-linked withvarying amounts of one or more cross-linking agents (e.g., divinylbenzene). If beads are constructed with a high degree of cross-linking,e.g., greater than 5 mol % cross-linker, it may be desirable toincorporate a porogen (e.g., toluene or cyclohexane) during the reactionused to construct the beads. The use of a porogen in such a way leavespermanent pores within the matrix constituting each bead. These poresmay be sufficiently large to allow the ingress of quantum dots into thebead.

Incorporating Quantum Dots into Prefabricated Beads

In respect of the second option for incorporating quantum dots into theprimary particles, the quantum dots may be immobilised within theprimary matrix material through physical entrapment. For example, asolution of quantum dots in a suitable solvent (e.g., an organicsolvent) may be incubated with a sample of primary particles. Removal ofthe solvent using any appropriate method results in the quantum dotsbecoming immobilised within the primary matrix material of the primaryparticles. The quantum dots remain immobilised in the particles unlessthe sample is resuspended in a solvent (e.g., an organic solvent) inwhich the quantum dots are freely soluble. One or morestability-enhancing additives may, for example, be included in thequantum dot solution which is incubated with the primary particles.Alternatively, the quantum dots may first be added to the primaryparticles, and the one or more additives then added to the primaryparticles. Additionally, at this stage, a surface coating may be appliedto the primary particles if desired.

In a further preferred embodiment, at least a portion of thesemiconductor nanoparticles may be physically attached to theprefabricated primary particles. Attachment may be achieved byimmobilisation of a portion of the semiconductor nanoparticles withinthe polymer matrix of the prefabricated primary particles or bychemical, covalent, ionic, or physical connection between thesemiconductor nanoparticles and the prefabricated primary particles. Ina particularly preferred embodiment the prefabricated primary particlesinclude polystyrene, polydivinyl benzene and a polythiol.

Quantum dots may be irreversibly incorporated into prefabricated primaryparticles in a number of ways, e.g., chemical, covalent, ionic, physical(e.g., by entrapment) or any other form of interaction. If prefabricatedprimary particles are to be used for the incorporation of quantum dots,the solvent accessible surfaces of the primary particles may bechemically inert (e.g., polystyrene) or alternatively they may bechemically reactive/functionalised (e.g., Merrifield's Resin). Thechemical functionality may be introduced during the construction of theprimary particles, for example by the incorporation of a chemicallyfunctionalised monomer, or alternatively, chemical functionality may beintroduced in a post-particle construction treatment step, for exampleby conducting a chloromethylation reaction. Additionally chemicalfunctionality may be introduced by a post-particle construction stepinvolving a polymeric graft or other similar process whereby chemicallyreactive polymer(s) are attached to the outer layers/accessible surfacesof the bead. More than one such post construction derivatisation processmay be carried out to introduce chemical functionality onto/into theprimary particles.

As with quantum dot incorporation into primary particles during theparticle forming reaction (i.e., the first option described above) thepre-fabricated primary particles may be of any shape, size andcomposition and may have any degree of cross-linker and may containpermanent pores if constructed in the presence of a porogen. Quantumdots may be imbibed into the primary particles by incubating a solutionof quantum dots in an organic solvent and adding this solvent to theprimary particles. The solvent is preferably capable of wetting theprimary particles, and in the case of lightly crosslinked primaryparticles, preferably 0-10% crosslinked and most preferably 0-2%crosslinked. The solvent may cause the polymer matrix to swell inaddition to solvating the quantum dots. Once the quantum dot-containingsolvent has been incubated with the primary particles, it may beremoved, for example by heating the mixture and causing the solvent toevaporate and the quantum dots to become embedded in the primary matrixmaterial constituting the primary particles, or alternatively, by theaddition of a second solvent in which the quantum dots are not readilysoluble but which mixes with the first solvent causing the quantum dotsto precipitate within the primary matrix material. Immobilisation may bereversible if the primary particles are not chemically reactive, orelse, if the primary particles are chemically reactive, the quantum dotsmay be held permanently within the primary matrix material, by chemical,covalent, ionic, or any other form of interaction. Any desirablestability-enhancing additive may be added during any of the stages ofthe quantum dot-bead fabrication described above.

Incorporation of Quantum Dots into Sol-Gels to produce Glass

As stated above, a preferred primary matrix material is an opticallytransparent media, such as a sol-gel or a glass. Such primary matrixmaterials may be formed in an analogous fashion to the method used toincorporate quantum dots into primary particles during the particleforming process as described above. For example, a single type ofquantum dot (e.g., one colour) may be added to a reaction mixture usedto produce a sol-gel or glass. Alternatively, two or more types ofquantum dot (e.g., two or more colours) may be added to a reactionmixture used to produce a sol-gel or glass. The sol-gels and glassesproduced by these procedures may have any shape, morphology or3-dimensional structure. For example, the resulting primary particlesmay be spherical, disc-like, rod-like, ovoid, cubic, rectangular, or anyof many other possible configurations. Any of the stability-enhancingadditives described hereinbefore may be added to quantum dot-containingglass beads. Some silica-based beads exhibit relatively low porosity ascompared, for example, to polymeric resin beads (e.g., acrylate-basedbeads). It may therefore be advantageous to add the or each additiveduring initial bead formation when the beads are made from asilica-based material rather than adding the additive(s) after beadformation, which may be more advantageous or desirable when using moreporous bead materials.

Application of Optional Surface Coating

In a preferred embodiment, where it is desired to provide a surfacecoating including an inorganic material on the quantum dot-containingprimary particles, such as a metal oxide or metal nitride, aparticularly preferred process to deposit the coating is atomic layerdeposition (ALD), although it will be appreciated that other suitabletechniques may be employed.

The provision of a surface coating by ALD, using a metal oxide surfacecoating as an example, typically includes the following four basicsteps:

1) Exposing a surface of a quantum dot-containing primary particle to ametal precursor;

2) Purging the reaction chamber containing the primary particles toremove non-reacted metal precursor and any gaseous reaction by-products;

3) Exposing the surface of the primary particles to an oxide precursor;and

4) Purging the reaction chamber.

The above steps may then be repeated any desirable number of times toprovide a surface coating of the desired thickness, for example, athickness of around 1 to 500 nm. Each reaction cycle adds apredetermined amount of coating material to the surface of the primaryparticles. One cycle may take from around 0.5 seconds to around 2-3seconds and deposit between 1 and 30 nm of surface coating.

Before initiating the ALD process, it is preferred that the surface ofthe primary particles be heat treated to ensure their stability duringthe ALD process. It will be appreciated that since ALD is essentially asurface-controlled process, where process parameters other than theprecursors, substrate (i.e., primary particle material), reactiontemperature (typically around 100 to 400° C., but may be as high as 500°C.), and, to a lesser extent pressure (typically around 1 to 10 mbar),have little or no influence on the final surface coating, ALD-grownsurface layers or films are extremely conformal and uniform inthickness, making ALD is a particularly preferred method for depositingprotective coatings on to the surface of quantum dot-containing primaryparticles.

A particularly preferred surface coating is Al₂O₃. An Al₂O₃ surfacecoating of only up to around 20 to 30 nm applied by ALD at a temperatureof around 100 to 175° C. using trimethylaluminium and water asprecursors may exhibit a very low water vapour transmission rate andpermeability to other gases and liquids.

It has been determined that ALD coatings applied to quantumdot-containing primary particles often result in the deposition of agreater quantity of the surface coating material, e.g., Al₂O₃ than wouldbe anticipated if the only surface being coated was the external surfaceof the primary particle. It has been established that an improvement inthe level or protection afforded by the surface coating may be achievedby increasing the amount of surface coating material deposited beyondthe amount theoretically required to coat just the calculated externalsurface area. While the inventors do not wish to be bound by anyparticular theory, it is believed that this is at least partly due tothe ALD process coating not just the external surface area of theprimary particles, but that it deposits coating material on at leastsome, if not substantially all of the accessible or effective surfacearea of the primary particle which includes internal voids that areaccessible from the outside of the primary particle. Thus, when porous,and particularly when highly porous polymeric bead-type materials arecoated using ALD, it has been observed that the coating material isdeposited inside the voids and pores of the primary particles, as wellas the outermost surface of the particles. In this way, the ALD processmay be used to reduce the porosity of the quantum dot-containing primaryparticles to unexpectedly and surprisingly low levels, thereby providinga degree of protection to the particles which is beyond that which wouldhave been anticipated by the skilled person. This has importantconsequences in terms of the processibility and optical performance ofthe final coated quantum dot-containing primary particles, both of whichmay be greatly enhanced compared to prior art quantum dot-basedmaterials, by the use of ALD to provide a surface coating of, forexample, Al₂O₃.

By way of example, it is known that heat treatment of prior art quantumdot-containing materials to the temperatures typically required duringLED manufacture (200° C. and above) degrades the performance of thematerials to unacceptably low levels. Moreover, the ability tophotobrighten such materials is also significantly diminished oreffectively lost following heat treatment. This places seriouslimitations on the use of quantum dot-based materials in applicationssuch as LED fabrication, as well as other manufacturing processesinvolving heat treatment of components. Aspects of the presentinvention, however, offer convenient solutions to these problems withprior art materials and methods. By the use of ALD to deposit a coatingmaterial, such as but not limited to Al₂O₃, to primary particlescontaining quantum dots, the coated materials may be heat treated attemperatures of up to at least 250° C. Not only do the materials remainstructurally sound but they may also be photobleached to substantiallyrestore their original quantum emission (i.e., before coating and heattreatment).

In an alternative preferred embodiment, the surface coating may beproduced in-situ on the surface of the primary particles. By way ofexample, a surface of quantum dot-containing primary particles may becontacted by polymerisable monomers which are then polymerised on thesurface of the particles to produce a polymeric surface coating on theparticles. One method by which contacting of the particles by themonomers may be effected is to disperse the particles within a monomermixture, optionally including a crosslinking agent and, if necessary, apolymerisation initiator, such as a photoinitiator. Polymerisation maythen be effected in any manner appropriate for the monomers being used.For example, if photopolymerisable monomers are used, then the polymermixture containing the primary particles and the optional photoinitiatormay then be exposed to a suitable source of radiation (e.g., UV).

FIGS. 11 to 14 depict alternative preferred arrangements of quantumdot-containing primary particles provided directly or indirectly with aprotective surface coating.

FIG. 11 illustrates a population of quantum dots entrapped within aprimary particle in the form of a polymer bead according to a preferredembodiment of the present invention. The primary particle is providedwith a surface coating of an inorganic material, before being dispersedwithin a secondary matrix material in the form of an LED encapsulantdisposed on an LED to provide a light-emitting device according to apreferred embodiment of the present invention. FIG. 12 depicts apopulation of quantum dots entrapped within a primary particle in theform of a polymer bead made from a first type of polymer (polymer 1)which is encapsulated within a second type of polymer material (polymer2). The surface of the second type of polymer is provided with aprotective surface coating of an inorganic material according to apreferred embodiment of the present invention. The encapsulated primaryparticles are then dispersed within a secondary matrix material in theform of an LED encapsulant disposed on an LED to provide alight-emitting device according to a preferred embodiment of the presentinvention. FIG. 13 illustrates a population of quantum dots entrappedwithin a population of primary particles in the form of polymer beads(bead 1) according to a preferred embodiment of the present invention inwhich each of the primary particles is provided with a surface coatingof an inorganic material. The coated primary particles are showndispersed within a second type of bead (bead 2) to produce a“bead-in-bead” composite material, which may be dispersed, as shown,within a secondary matrix material in the form of an LED encapsulantdisposed on an LED to provide a light-emitting device according to apreferred embodiment of the present invention. FIG. 14 depicts apopulation of quantum dots entrapped within a population of primaryparticles in the form of polymer beads according to a preferredembodiment of the present invention, the population of primary particlesbeing dispersing within a second type of bead to produce a“bead-in-bead” composite material which is then provided with aninorganic surface coating layer. The coated bead-in-bead compositematerial may then be dispersed within a secondary matrix material asshown in the form of an LED encapsulant disposed on an LED to provide alight-emitting device according to a preferred embodiment of the presentinvention.

Application of QD-Beads—

Incorporation into LED Encapsulant

While the addition of one or more additives to beads containing quantumdots has many advantages as outlined above, one significant advantage ofembodiments of the present invention is that additive-containing quantumdot-beads (QD-beads) produced as described above may be incorporatedinto commercially available LED encapsulant materials simply by weighingthe desired amount of the QD-bead material and adding this to thedesired amount of LED encapsulant material.

It is preferred that the bead/encapsulant composite be mixed thoroughlyto provide a homogeneous mixture. The mixture may then be dispensed ontoa commercially available LED and cured according to normal curingprocedures for the particular LED-encapsulant used. Theadditive-containing QD-beads thus provide a simple and straightforwardway of facilitating the formulation of bead/LED encapsulant compositeswhich may be used in the fabrication of next generation, higherperformance light-emitting devices using, as far as possible, standardcommercially available materials and methods.

LED Encapsulating Materials

Any existing commercially available LED encapsulant may be used withadditive-containing QD-beads produced according to embodiments of thepresent invention. Preferred LED encapsulants include silicones,epoxies, (meth)acrylates and other polymers, although it will beappreciated by the skilled person that further options are available,such as but not limited to silica glass, silica gel, siloxane, sol gel,hydrogel, agarose, cellulose, epoxy, polyether, polyethylene, polyvinyl,poly-diacetylene, polyphenylene-vinylene, polystyrene, polypyrrole,polyimide, polyimidazole, polysulfone, polythiophene, polyphosphate,poly(meth)acrylate, polyacrylamide, polypeptide, polysaccharide andcombinations thereof.

LED encapsulants which may be used include, but are not limited to, UVcurable encapsulants and heat-curable encapsulants, includingencapsulants which require one or more catalysts to support the curingprocess. Specific examples of commercially available siliconeencapsulants which are suitable may be selected from the groupconsisting of SCR1011, SCR1012, SCR1016. LPS-3412 (all available fromShin Etsu) and examples of suitable epoxy encapsulants may be selectedfrom the group consisting of Pacific Polytech PT1002, Fine PolymersEpifine EX-1035A, and Fine Polymers Epifine X-1987.

Colour Indexing

The colour of the light output from an additive-containing QD-bead-LED(the “secondary light”) may be measured using a spectrometer. Thespectral output (mW/nm) may then be processed mathematically so that theparticular colour of the light-emitting device may be expressed ascolour coordinates on a chromaticity diagram, for example the 2° CIE1931 chromaticity diagram (see FIG. 2).

The 2° CIE 1931 chromaticity coordinates for a particular spectrum maybe calculated from the spectral power distribution and the CIE 1931colour matching functions x, y, z (see FIG. 3). The correspondingtristimulus values may be calculated thusX=∫pxdλ Y=∫pydλ Z=∫pzdλ

Where p is the spectral power, and x, y and z are the colour matchingfunctions.

From X, Y, and Z the chromaticity coordinates x, y may be calculatedaccording to

$x = {{\frac{X}{X + Y + Z}\mspace{14mu}{and}\mspace{14mu} y} = \frac{Y}{X + Y + Z}}$

Using x, y as the coordinates, a two-dimensional chromaticity diagram(the CIE 1931 colour space diagram) may be plotted which is analogous tothe exemplary diagram depicted in FIG. 2.

Colour Rendering

Colour rendering describes the ability of a light source to illuminateobjects such that they appear the correct colour when compared to howthey appear when illuminated by a reference light source. Usually thereference light source is a tungsten filament bulb which is assigned acolour rendering index (CRI) of 100. To be acceptable for generallighting, a white light-emitting device source typically has a CRI>80.An example of poor colour rendering is the sodium street lamp which hasvery poor colour rendering capability, i.e., it is difficult todistinguish a red car from a yellow car illuminated by a sodium lamp, inthe dark under a sodium lamp they both appear grey.

Embodiments of the present invention provide a plurality of robust,high-performance additive-containing QD-beads which may be used tofabricate a light-emitting device. The quantum dots within the primaryparticles or beads generally are in optical communication with a primarysolid-state photon/light source (e.g., an LED, laser, arc lamp orblack-body light source) such that, upon excitation by primary lightfrom the primary light source the quantum dots within the primaryparticles emit secondary light of a desired colour. The requiredintensities and emission wavelengths of the light emitted from thedevice itself may be selected according to appropriate mixing of thecolour of the primary light with that of the secondary light(s) producedfrom the down-conversion of the primary light by the quantum dots.Moreover, the size (and thus emission) and number of each type ofquantum dot within the primary particles may be controlled, as may thesize, morphology and constituency of the primary matrix material makingup the primary particles, such that subsequent mixing of the quantumdot-containing media allows light of any particular colour and intensityto be produced.

It will be appreciated that the overall light emitted from the devicemay consist effectively of the light emitted from the quantum dots,i.e., just the secondary light, or a mixture of light emitted from thequantum dots and light emitted from the solid-state/primary lightsource, i.e., a mixture of the primary and secondary light. Colourmixing of the quantum dots may be achieved either within the quantumdot-containing media (e.g., within each bead in a population of beadssuch that each bead contains a number of different size/colour emittingquantum dots) or a mixture of differently coloured primary matrixmaterials with all the quantum dots within a specific matrix materialbeing the same size/colour (e.g., some beads containing all greenquantum dots and others containing all red quantum dots).

EXAMPLES

Examples 1 and 2 below describe the preparation of additive-containingquantum dot beads, and Example 3 describes how to coat such beads, whichcould, for example, be in used in the fabrication of new, improvedquantum dot-based light-emitting devices. The Synthetic Methods sectionprovides two methods for producing quantum dots (1.1 and 1.2) and threemethods for incorporating quantum dots into primary particles or “beads”(2.1, 2.2 and 2.3).

Synthetic Methods

1.1 Preparation of CdSe/ZnS Core/Shell Quantum Dots

Preparation of CdSe Cores

HDA (500 g) was placed in a three-neck round bottomed flask and driedand degassed by heating to 120° C. under a dynamic vacuum for >1 hour.The solution was then cooled to 60° C. To this was added 0.718 g of[HNEt₃]₄[Cd₁₀Se₄(SPh)₁₆] (0.20 mmols). In total 42 mmols, 22.0 ml ofTOPSe and 42 mmols, (19.5 ml, 2.15 M) of Me₂Cd.TOP was used. Initially 4mmol of TOPSe and 4 mmols of Me₂Cd.TOP were added to the reaction atroom temperature and the temperature increased to 110° C. and allowed tostir for 2 hours. The reaction was a deep yellow colour, the temperaturewas progressively increased at a rate of ˜1° C./5 min with equimolaramounts of TOPSe and Me₂Cd.TOP being added dropwise. The reaction wasstopped when the PL emission maximum had reached ˜600 nm, by cooling to60° C. followed by addition of 300 ml of dry ethanol or acetone. Thisproduced a precipitation of deep red particles, which were furtherisolated by filtration. The resulting CdSe particles were recrystallizedby re-dissolving in toluene followed by filtering through Celitefollowed by re-precipitation from warm ethanol to remove any excess HDA,selenium or cadmium present. This produced 10.10 g of HDA capped CdSenanoparticles. Elemental analysis C=20.88, H=3.58, N=1.29, Cd=46.43%.Max PL=585 nm, FWHM=35 nm. 38.98 mmols, 93% of Me₂Cd consumed in formingthe quantum dots.

Growth of ZnS Shell

HDA (800 g) was placed in a three neck round-bottom flask, dried anddegassed by heating to 120° C. under a dynamic vacuum for >1 hour. Thesolution was then cooled to 60° C. To this was added 9.23 g of CdSenanoparticles that have a PL maximum emission of 585 nm. The HDA wasthen heated to 220° C. To this was added by alternate dropwise additiona total of 20 ml of 0.5 M Me₂Zn.TOP and 0.5 M, 20 ml of sulfur dissolvedin octylamine. Three alternate additions of 3.5, 5.5 and 11.0 ml of eachwere made, whereby initially 3.5 ml of sulphur was added dropwise untilthe intensity of the PL maximum was near zero. Then 3.5 ml of Me₂Zn.TOPwas added dropwise until the intensity of the PL maximum had reached amaximum. This cycle was repeated with the PL maximum reaching a higherintensity with each cycle. On the last cycle, additional precursor wasadded once the PL maximum intensity been reached until it was between5-10% below the maximum intensity, and the reaction was allowed toanneal at 150° C. for 1 hour. The reaction mixture was then allowed tocool to 60° C. whereupon 300 ml of dry “warm” ethanol was added whichresulted in the precipitation of particles. The resulting CdSe—ZnSparticles were dried before re-dissolving in toluene and filteringthrough Celite followed by re-precipitation from warm ethanol to removeany excess HDA. This produced 12.08 g of HDA capped CdSe—ZnS core-shellnanoparticles. Elemental analysis C=20.27, H=3.37, N=1.25, Cd=40.11,Zn=4.43%; Max PL 590 nm, FWHM 36 nm.

1.2 Preparation of InP/ZnS Core/Shell Quantum Dots

Preparation of InP Cores (500-700 nm emission)

Di-butyl ester (100 ml) and Myristic acid (10.0627 g) were placed in athree-neck flask and degassed at 70° C. under vacuum for one hour. Afterthis period, nitrogen was introduced and the temperature increased to90° C. ZnS molecular cluster [Et₃NH₄][Zn₁₀S₄(SPh)₁₆] (4.7076 g) wasadded and the mixture allowed to stir for 45 minutes. The temperaturewas then increased to 100° C. followed by the dropwise addition ofIn(MA)₃ (1 M, 15 ml) followed by (TMS)₃P (1 M, 15 ml). The reactionmixture was allowed to stir while increasing the temperature to 140° C.At 140° C., further dropwise additions of In(MA)₃ (1 M, 35 ml) (left tostir for 5 minutes) and (TMS)₃P (1 M, 35 ml) were made. The temperaturewas then slowly increased to 180° C. and further dropwise additions ofIn(MA)₃ (1 M, 55 ml) followed by (TMS)₃P (1 M, 40 ml) were made. Byaddition of the precursor in the manner described above, nanoparticlesof InP could be grown with the emission maximum gradually increasingfrom 520 nm up to 700 nm, whereby the reaction may be stopped when thedesired emission maximum has been obtained and left to stir at thistemperature for half an hour. After this period, the temperature wasdecreased to 160° C. and the reaction mixture was left to anneal for upto 4 days (at a temperature between 20-40° C. below that of thereaction). A UV lamp was also used at this stage to aid in annealing.

The nanoparticles were isolated by the addition of dried degassedmethanol (approx. 200 ml) via cannula techniques. The precipitate wasallowed to settle and then methanol was removed via cannula with the aidof a filter stick. Dried degassed chloroform (approx. 10 ml) was addedto wash the solid. The solid was left to dry under vacuum for 1 day.This produced 5.60 g of InP core nanoparticles. Elemental analysis: maxPL=630 nm, FWHM=70 nm.

Post-Operative Treatments

The quantum yields of the InP quantum dots prepared above were increasedby washing with dilute HF acid. The dots were dissolved in anhydrousdegassed chloroform (˜270 ml). A 50 ml portion was removed and placed ina plastic flask, flushed with nitrogen. Using a plastic syringe, the HFsolution was made up by adding 3 ml of 60% w/w HF in water and adding todegassed THF (17 ml). The HF was added dropwise over 5 hrs to the InPdots. After addition was complete the solution was left to stirovernight. Excess HF was removed by extracting through calcium chloridesolution in water and drying the etched InP dots. The dried dots werere-dispersed in 50 ml chloroform for future use. Max 567 nm, FWHM 60 nm.The quantum efficiencies of the core materials at this stage range from25-90%.

Growth of ZnS Shell

A 20 ml portion of the HF etched InP core particles was dried down in a3-neck flask. 1.3 g myristic acid and 20 ml di-n-butyl sebacate esterwas added and degassed for 30 minutes. The solution was heated to 200°C., then 1.2 g anhydrous zinc acetate was added and 2 ml 1 M (TMS)₂S wasadded dropwise (at a rate of 7.93 ml/hr) after addition was complete thesolution was left to stir. The solution was kept at 200° C. for 1 hr,then cooled to room temperature. The particles were isolated by adding40 ml of anhydrous degassed methanol and centrifuged. The supernatantliquid was disposed of and to the remaining solid 30 ml of anhydrousdegassed hexane was added. The solution was allowed to settle for 5 hrsand then re-centrifuged. The supernatant liquid was collected and theremaining solid was discarded. PL emission peak Max.=535 nm, FWHM=65 nm.The quantum efficiencies of the nanoparticle core/shell materials atthis stage ranged from 35-90%.

2.1 Incorporating Quantum Dots into Suspension Polymeric Beads

1% wt/vol polyvinyl acetate (PVA) (aq) solution was prepared by stirringfor 12 hours followed by extensive degassing by bubbling nitrogenthrough the solution for a minimum of 1 hour. The monomers, methylmethacrylate and ethylene glycol dimethacrylate, were also degassed bynitrogen bubbling and used with no further purification. The initiatorAIBN (0.012 g) was placed into the reaction vessel and put under threevacuum/nitrogen cycles to ensure no oxygen was present.

CdSe/ZnS core/shell quantum dots as prepared above in Method 1 wereadded to the reaction vessel as a solution in toluene and the solventremoved under reduced pressure. Degassed methyl methacrylate (0.98 mL)was then added followed by degassed ethylene glycol dimethacrylate (0.15mL). The mixture was then stirred at 800 rpm for 15 minutes to ensurecomplete dispersion of the quantum dots within the monomer mixture. Thesolution of 1% PVA (10 mL) was then added and the reaction stirred for10 minutes to ensure the formation of the suspension. The temperaturewas then raised to 72° C. and the reaction allowed to proceed for 12hours.

The reaction mixture was then cooled to room temperature and the beadedproduct washed with water until the washings ran clear followed bymethanol (100 mL), methanol/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/dichloromethane (1:1, 100 mL),dichloromethane (100 mL), dichloromethane/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/methanol (1:1, 100 mL),methanol (100 mL). The quantum dot-containing beads (QD-beads) were thendried under vacuum and stored under nitrogen.

2.2 Adsorbing Quantum Dots into Prefabricated Beads

Polystyrene microspheres with 1% divinyl benzene (DVB) and 1% thiolco-monomer were resuspended in toluene (1 mL) by shaking and sonication.The microspheres were centrifuged (6000 rpm, approx 1 min) and thesupernatant decanted. This was repeated for a second wash with tolueneand the pellets then resuspended in toluene (1 mL).

InP/ZnS quantum dots as prepared above in Method 2 were dissolved (anexcess, usually 5 mg for 50 mg of microspheres) in chloroform (0.5 mL)and filtered to remove any insoluble material. The quantumdot-chloroform solution was added to the microspheres in toluene andshaken on a shaker plate at room temperature for 16 hours to mixthoroughly.

The quantum dot-microspheres were centrifuged to pellet and thesupernatant decanted off, which contained any excess quantum dotspresent. The pellet was washed (as above) twice with toluene (2 mL),resuspended in toluene (2 mL), and then transferred directly to glasssample vials used in an integrating sphere. The glass vials werepelleted down by placing the vials inside a centrifuge tube,centrifuging and decanting off excess toluene. This was repeated untilall of the material had been transferred into the sample vial. A quantumyield analysis was then run directly on the pellet, wet with toluene.

2.3 Incorporation of Quantum Dots into Silica Beads

0.4 mL of InP/ZnS core shell quantum dots capped with myristic acid(around 70 mg of inorganic) was dried under vacuum. 0.1 mL of(3-(trimethoxysilyl)propyl methacrylate (TMOPMA), followed by 0.5 mL oftriethylorthosilicate (TEOS) was injected to dissolve the dried quantumdots and form a clear solution, which was kept for incubation under N₂overnight. The mixture was then injected into 10 mL of a reversemicroemulsion (cyclohexane/CO-520, 18 ml/1.35 g) in 50 mL flask, understirring @ 600 rpm. The mixture was stirred for 15 mins and then 0.1 mLof 4% NH₄OH was injected to start the bead forming reaction. Thereaction was stopped the next day by centrifugation to collect the solidphase. The obtained particles were washed twice with 20 mL cyclohexaneand then dried under vacuum.

Example 1 Addition of Additive(s) to QD-Containing Beads

Any of the stability-enhancing additives set out hereinbefore may beadded to a quantum dot solution before processing the solution intobeads (e.g., mixed with a suitable monomer, crosslinker, and optionallyother ingredients), or added later to the pre-formed beads by incubationinto a solution of the additive, i.e., soaking, for a suitable period oftime. Soaking procedures for the addition of additives to pre-formedbeads involved adding 30 mg of dried quantum dot-containing beads to oneor more additive solutions in ethanol (additive concentration=1 mM). Thebeads were then incubated in this mixture for 30 mins and then dried byvacuum.

Example 2 Addition of Additive(s) to QD-Containing Beads Contained inLarger Beads

Inner beads containing quantum dots were mixed with one or moreadditives and then embedded within a larger bead. The final“bead-in-bead” material was then treated by soaking as described inExample 1.

Example 3 Coating Quantum Dot-Containing Silica Beads withPolymethylmethacrylate

25 mg powdered quantum dots-containing silica beads was dispersed aswell as possible in degassed methylmethacrylate (MMA). A photoinitiator,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, was added to acrosslinker, trimethylolpropanetrimethacrylate (TMPTM), and dissolvedwhile the solution was degassed. The TMPTM crosslinker was then added tothe MMA and silica and the mixture agitated on a whirlmixer to ensurehomogeneous mixing of the monomer and crosslinker. The resulting slurrywas transferred to a syringe with a wide bore needle and thencontinuously agitated while being injected into 5 mL of degassed 2% PVAstirring at 1200 rpm. The suspension was then exposed to 365 nm UV lightfor 30 minutes. The mixture was stirred overnight and worked-up thefollowing morning by washing and centrifugation. Washes of 2×20 mL ofH₂O and 2×20 mL EtOH and centrifugation of 2000 rpm for 2 mins betweenwashes. The sample was finally dried under vacuum and purged with N₂.

Example 4 Coating Quantum Dot-Containing Silica Beads withPolymethylmethacrylate

25 mg powdered quantum dots-containing silica beads was dispersed aswell as possible in degassed methylmethacrylate (MMA). A photoinitiator,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, was added to acrosslinker, trimethylolpropanetrimethacrylate (TMPTM), and dissolvedwhile the solution was degassed. The TMPTM crosslinker was then added tothe MMA and silica and the mixture agitated on a whirlmixer to ensurehomogeneous mixing of the monomer and crosslinker. The resulting slurrywas transferred to a syringe with a wide bore needle and thencontinuously agitated while being injected into 5 mL of degassed 2% PVAstirring at 1200 rpm. The suspension was then exposed to 365 nm UV lightfor 30 minutes. The mixture was stirred overnight and worked-up thefollowing morning by washing and centrifugation. Washes of 2×20 mL ofH₂O and 2×20 mL EtOH and centrifugation of 2000 rpm for 2 mins betweenwashes. The sample was finally dried under vacuum and purged with N₂.

Example 5 GelestHardsil AR Coating Polymeric Bead Procedure

A stock solution of GelestHardsil AR (2000 μl) and Zn-2-ethylhexanoate(10 μl) was made. Under a N_(2(g)) atmosphere, an aliquot (150 μL) ofthe GelestHardsil AR/Zn-2-ethylhexanoate stock solution was added toCFQD-Beads (30 mg) in a glass vial (˜7 mL), incubated (overnight),placed under high vacuum (overnight) and a screw cap lid fitted to thevial. The sample was removed the glove box and placed in a preheated(220° C.) heating block mounted on a hot plate (20 min).

Example 6 Poly(Vinylalcohol) Coating Quantum Dot-Beads Procedure

A stock solution of poly(vinylalcohol) (87-89% hydrolysed,MW=85000-124000) (0.05 g) dissolved (100° C.) in ethyleneglycol (5 ml)was made. Under a N_(2(g)) atmosphere, an aliquot (150 μL) of thepoly(vinylalcohol)/ethyleneglycol stock solution was added to quantumdot beads, mixed and placed under high vacuum overnight to give a drypowder (QY=35%, PL=527 nm, FWHM=70 nm).

Example 7 Polymerisation Coating of Quantum Dot Beads withGlycidylmethacrylate with BF₃ & UV-Light

Under a N_(2(g)) atmosphere, a stock solution of glycidyl methacrylate(inhibitor removed) (1406 μl), BF₃.OEt₂ (12.3 μl) and Irgacure 819 (9mg) was made. An aliquot (100 μl) was of the stock solution was added toquantum dot-beads (20 mg), mixed and irradiated with UV-LED (HamamatsuUV-LED, 3 min). The samples were returned to the glove box to allow theepoxide polymerisation to proceed.

Example 8 Optocast Coating Quantum Dot-Bead Procedure

A stock solution of epoxy resin (Optocast 3553, 30 μl) dissolved indiethylether (1470 μl) was made. Under a N_(2(g)) atmosphere, an aliquot(150 μL) of the Optocast/diethylether stock solution was added toquantum dot beads (30 mg), mixed, incubated (1.5 hr), placed under highvacuum overnight and irradiated (Hg-lamp, 400 W, 5 min) to give aparticles (QY=30%, PL=515 nm, FWHM=70 nm).

It will be seen that the techniques described herein provide a basis forimproved production of nanoparticle materials. The terms and expressionsemployed herein are used as terms of description and not of limitation,and there is no intention in the use of such terms of and expressions ofexcluding any equivalents of the features shown and described orportions thereof. Instead, it is recognized that various modificationsare possible within the scope of the invention claimed.

What is claimed is:
 1. A composite material comprising: (a) a particlecomprising: a microbead comprising a primary matrix material; apopulation of semiconductor nanoparticles contained within themicrobead, the nanoparticles comprising at least one surface-boundligand; a first additive to enhance the physical, chemical and/orphoto-stability of the semiconductor nanoparticles and, a surfacecoating disposed on the surface of the microbead; wherein, the primarymatrix material, the surface bound ligand, the additive, and the surfacecoating are different materials; and (b) a secondary matrix material inthe form of one or more secondary particles containing one or moresecondary additives to enhance the physical, chemical and/orphoto-stability of the semiconductor nanoparticles, wherein (c) theparticle of (a) is dispersed within the secondary matrix material of (b)and wherein the first additive and the one or more secondary additivesare the same or different.
 2. A composite material according to claim 1,wherein said microbeads possess an average diameter of around 20 nm toaround 0.5 mm.
 3. A composite material according to claim 1, whereinsaid primary matrix material comprises a material selected from thegroup consisting of a polymer, a resin, a monolith, a glass, a sol gel,an epoxy, a silicone, and a (meth)acrylate.
 4. A composite materialaccording to claim 1, wherein said first additive mechanically seals theprimary matrix material.
 5. A composite material according to claim 1,wherein said first additive reduces a porosity of the primary matrixmaterial.
 6. A composite material according to claim 4, wherein saidfirst additive is selected from the group consisting of fumed silica,ZnO, TiO₂, ZrO, Mg stearate, and Zn stearate.
 7. A composite materialaccording to claim 1, wherein said first additive is a capping agent. 8.A composite material according to claim 7, wherein said first additiveis selected from the group consisting of tetradecyl phosphonic acid,oleic acid, stearic acid, polyunsaturated fatty acids, sorbic acid, Znmethacrylate, Mg stearate, Zn stearate, and isopropyl myristate.
 9. Acomposite material according to claim 1, wherein said surface coatingcomprises a material providing the primary particle with a protectivebarrier to prevent the passage or diffusion of potentially deleteriousspecies from the external environment through the matrix material to thesemiconductor nanoparticles.
 10. A composite material according to claim1, wherein the surface coating is a barrier that hinders or prevents thepassage of oxygen or an oxidizing agent through the primary matrixmaterial.
 11. A composite material according to claim 1, wherein thesurface coating is a barrier that hinders or prevents the passage offree radical species through the matrix material.
 12. A compositematerial according to claim 1, wherein the surface coating is a barrierthat hinders or prevents moisture passing through the matrix material.13. A composite material according to claim 1, wherein the surfacecoating has a thickness of up to around 500 nm.
 14. A composite materialaccording to claim 13, wherein the thickness of the surface coating isselected from the range of around 5 to 100 nm.
 15. A composite materialaccording to claim 1, wherein the surface coating comprises an inorganicmaterial.
 16. A composite material according to claim 15, wherein theinorganic material is selected from the group consisting of adielectric, a metal oxide, a metal nitride, and a silica-based material.17. A composite material according to claim 1, wherein the surfacecoating comprises a polymeric material.
 18. A composite materialaccording to claim 17, wherein the polymeric material is a saturated orunsaturated hydrocarbon polymer, or is a polymer incorporating one ormore heteroatoms or heteroatom-containing functional groups.
 19. Acomposite material according to claim 1, wherein said semiconductornanoparticles contain ions selected from group 11, 12, 13, 14, 15 and/or16 of the periodic table, or one or more types of transition metal ionor d-block metal ion.
 20. A composite material according to claim 1,wherein said secondary matrix material comprises a material selectedfrom the group consisting of a polymer, a resin, a monolith, a glass, asol gel, an epoxy, a silicone, and a (meth)acrylate.
 21. Alight-emitting device including a primary light source in opticalcommunication with a formulation comprising a composite materialaccording to claim 1 embedded in a host light-emitting diodeencapsulation medium.
 22. A composite material comprising: (a) aparticle comprising: a microbead comprising a primary matrix material; apopulation of semiconductor nanoparticles contained within themicrobead, the nanoparticles comprising at least one surface-boundligand; and a first additive to enhance the physical, chemical and/orphoto-stability of the semiconductor nanoparticles wherein, the primarymatrix material, the surface bound ligand and the first additive aredifferent materials; and (b) a secondary matrix material in the form ofone or more secondary particles containing one or more secondaryadditives to enhance the physical, chemical and/or photo-stability ofthe semiconductor nanoparticles, wherein the additive is selected fromthe group consisting of a reducing agent, a free radical scavenger, anda hydride reactive agent, and wherein (c) the particle of (a) isdispersed within the secondary matrix material of (b) and wherein thefirst additive and the secondary additives are the same or different.23. A composite material according to claim 22, wherein said firstadditive is selected from the group consisting of ascorbic acidpalmitate, alpha tocopherol, octane thiol, butylated hydroxyanisole,butylated hydroxy toluene, a gallate ester, and a metabisulfite.
 24. Acomposite material according to claim 22, wherein said first additive isa benzophenone.
 25. A composite material according to claim 22, whereinsaid first additive is selected from the group consisting of1,4-butandiol, 2-hydroxyethyl methacrylate, allyl methacrylate, 1,6heptadiene-4-01, 1,7 octadiene, and 1,4 butadiene.